When a convex lens of focal length 10 cm is placed in contact with a concave lens, the combination has a power of +5 diopter. The power of combination, when the lenses are separated by 10 cm in air, will be
A
+10D
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B
+20D
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C
−5D
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D
−30D
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Solution
The correct option is A+10D We know, 1f=1f1+1f2 where f is the focal length of the combination and f1 and f2 are the respective focal lengths of the two lenses.
Where, f=15m=20cm
∴120=110+1f2
⇒f2=−20cm
When lenses are separated, then 1f=1f1+1f2−df1f2, where d is the distance between two lenses.